1. Field of the Invention
The present invention relates to an imaging area specifying apparatus, a radiographic system, an imaging area specifying method, a radiographic apparatus, and an imaging table.
2. Description of the Related Art
In recent years, radiation detectors, such as flat panel detectors (FPD) that include a radiation sensitive layer provided on a thin film transistor (TFT) active matrix substrate, detect radiation, such as X-rays emitted, and output an electric signal indicating a radiological image represented by the detected radiation, have been put to practical use. The radiation detector has an advantage in that it enables the user to instantly check images including moving images, as compared to the X-ray film or the imaging plate according to the related art.
In addition, portable radiographic apparatuses (hereinafter, referred to as “electronic cassettes”) that include the radiation detector and store radiological image data output from the radiation detector have been put to practical use. Since the electronic cassette has high portability, it can capture the image of the patient lying on a stretcher or a bed as they are, and it is easy to adjust the position of an imaging portion by changing the position of the electronic cassette. Therefore, it is possible to capture the image of a patient who cannot move.
However, in the cassette according to the related art including X-ray film or an imaging plate (IP), imaging is performed with a cassette having a film or an imaging plate with a size corresponding to an imaging portion or an imaging technique. This is because radiography is performed only on an imaging portion that needs to be observed considering the exposure of the patient and the periphery thereof and it is reasonable to use a film with a size corresponding to an imaging portion and the periphery thereof. Therefore, in the related art, cassettes with plural sizes are prepared.
Meanwhile, the electronic cassette is more expensive than the cassette including the X-ray film or the imaging plate. In addition, digital radiological image data is obtained by radiography, and a so-called trimming process of validating only data in a specific area in the radiological image data is easily performed. Therefore, a single-size electronic cassette can respond to plural imaging portions and imaging techniques by capturing images using the entire detection region for detecting radiation, or emitting radiation only to an imaging portion which needs to be observed and the periphery thereof using a portion of the detection region and trimming the image of the irradiated portion.
Japanese Patent Application Laid-Open (JP-A) No. 2003-33343 discloses a technique that divides the detection region into plural regions and performs an imaging operation in each of the divided regions.
Japanese Patent No. 2716949 and JP-A No. 2009-17484 disclose a technique that changes the reading range of pixel information from the radiation detector to narrow the imaging area of a radiological image and limits a region irradiated with radiation in correspondence with the imaging area such that fluorography is performed at a high frame rate, in a fluorography mode that continuously captures the image of an imaging portion which rapidly moves, such as the heart, to obtain a moving image.
However, in the technique disclosed in JP-A No. 2003-33343, in some cases, a specific portion of the detection region of the radiation detector deteriorates.
In the radiation detector, the region to which radiation is emitted deteriorates. Therefore, in a case in which an imaging operation is repeatedly performed using a specific portion of the detection region of the radiation detector, only that portion deteriorates. For example, as shown in FIG. 16, in a case in which there is a mark (“+” mark) indicating the center of the detection region, in many cases, the center of the detection region is used to perform an imaging operation. However, when an imaging operation is repeatedly performed using a central portion of the detection region, the central portion deteriorates, and the quality of the radiological image captured in the central portion deteriorates. In addition, in a case in which an imaging operation is performed using the entire detection region of the radiation detector, there is a difference in quality between the images captured in the central portion and a peripheral portion.
In particular, in fluorography, the number of shots is large and the total amount of radiation emitted to the radiation detector is more than that in the general still image capture mode. Therefore, as in Japanese Patent No. 2716949 and JP-A No. 2009-17484, in a case in which the imaging area of the radiological image is narrowed and the region irradiated with radiation is limited in correspondence with the imaging area, the imaging area deteriorates. For example, in fluorography, in general, the imaging area is limited to the central portion of the radiation detector and the central portion deteriorates. However, Japanese Patent No. 2716949 and JP-A-2009-17484 do not disclose any measures to prevent the deterioration.
In recent years, radiation detectors, such as flat panel detectors (FPD) that include a radiation sensitive layer disposed on a thin film transistor (TFT) active matrix substrate and can directly convert radiation, such as X-rays emitted, into digital data, have been put to practical use. The radiation detector has an advantage in that it enables the user to instantly check images and performs the fluorography mode (moving image capture mode) which continuously captures images, as compared to the X-ray film or the imaging plate according to the related art. Radiation conversion methods performed in the radiation detector include, for example, an indirect conversion method of converting radiation into light using a scintillator and converting the light into charge using a semiconductor layer, such as a photodiode, and a direct conversion method of converting radiation into charge using a semiconductor layer made of, for example, amorphous selenium. In each of the methods, there are various kinds of materials that can be used for the semiconductor layer.
However, a region to which radiation is emitted in the radiation detector deteriorates. Examples of the deterioration include irreversible deterioration in which radiation is repeatedly emitted to the semiconductor layer and the semiconductor layer gradually deteriorates, which causes a defective pixel, and reversible deterioration in which, when radiation is repeatedly emitted to the same region in a short time in the moving image capture mode, residual charge is gradually stored and the quality of the image deteriorates. The irreversible deterioration is likely to occur when the semiconductor layer is made of amorphous selenium. The reversible deterioration occurs when charge is stored in the semiconductor layer in the direct conversion method, and occurs when charge is stored in a photoelectric conversion unit (photodiode) in the indirect conversion method. The reversible deterioration is removed over time or by a removal process of removing the residual charge. However, it is difficult to perform the removal process while a moving image is captured.
JP-A No. 2000-134539 discloses a technique for obtaining a good radiological image using a radiation detector with defective pixels. In the technique, in one radiographic operation, the radiation detector is moved each time the radiological image is read, thereby reading plural radiological images from the radiation detector, and the read plural radiological images overlap each other such that the image of an object overlaps, thereby generating the image data of the radiological image.
JP-A No. 2007-215760 discloses a technique that reads pixel data of a portion of the detection region according to the exposure field range of radiation when an imaging apparatus using the radiation detector captures a moving image, in order to improve the reading speed of image data when the moving image is captured.
However, in the techniques disclosed in JP-A No. 2000-134539 and JP-A No. 2007-215760, in some cases, a specific portion of the detection region of the radiation detector deteriorates.
When an imaging operation is repeatedly performed using a specific portion of the detection region of the radiation detector, only the portion deteriorates. For example, when an imaging operation is repeatedly performed using a central portion of the detection region, the central portion deteriorates, and the quality of the radiological image captured in the central portion deteriorates. In particular, in fluorography, the number of shots is large and the total amount of radiation emitted to the radiation detector is more than that in the general still image capture mode.